This invention relates generally to molding apparatus and, more particularly, to injection molding apparatus for producing centrally apertured record discs, such as video discs.
An example of a molding apparatus of this particular type is disclosed in commonly assigned U.S. Pat. Nos. 4,185,955 and 4,260,360 filed in the name of J. R. Holmes et al, and entitled "Method and Means for Replicating Centrally Apertured Video Disc Records". The apparatus described in that application is used in an injection molding machine and includes first and second mold halves reciprocally movable between a closed position, wherein molten disc material is injected into an annular cavity to form a video disc, and an open position, wherein the parting line, which passes through the cavity, is opened and the molded video disc can be removed from the apparatus. The annular cavity is defined by a pair of planar, disc-shaped stampers secured to platens on the respective mold halves.
After the injected disc material has cured, the molding machine begins moving the two mold halves from the closed position to the open position. A special latch lock mechanism in the apparatus holds the parting line initially closed, causing initial movement apart to occur between two plates on the first mold half. When the two plates have moved apart by a prescribed amount, the central aperture is punched in the molded video disc, after which time the latch lock mechanism permits the parting line to open and the video disc to be extracted.
Each stamper has a central opening and is secured at its inner periphery by a special center clamp and at its outer periphery by a special outer ring clamp. When molten material is injected into the cavity, air is vented through a narrow gap between the stamper for the second mold half and a radially inwardly projecting annular foot that is integral with the outer ring clamp for the first mold half. Although this arrangement is generally satisfactory in most situations, variations in stamper thicknesses can affect the size of the vent passage, thereby resulting in an undesired variation in the back pressure afforded by the venting air.
Another drawback to the molding apparatus described in the aforementioned patent application arises from the fact that the stampers can be replaced only by a delicate insertion procedure at the site of the apparatus. Because of a difficulty normally encountered in accurately positioning the stampers relative to the clamps, they can sometimes be damaged by inadvertent abrasion. Still another problem arises from the fact that the apparatus includes two stampers, even though information to be molded into the disc is ordinarily carried on only one of them, the other being a blank dummy. The dummy stamper, like the stamper that carries information, is similarly subject to damage from abrasion and from dimpling, which is caused by the presence of minute foreign particles lodged between the stamper and its underlying platen.
The two mold halves are aligned with respect to each other by means of a number of guide pins located on one mold half and engagable with corresponding bushings located in the other mold half. Although generally effective in maintaining the two mold halves properly aligned, the guide pins and bushings can sometimes experience wear, to the extent that a slight misalignment of the two mold halves can occur.
Still another drawback to the aforementioned molding apparatus arises from the use of the latch lock mechanism to keep the parting line closed until the central aperture has been punched in the molded disc. The mechanism includes a rotating latching member having a pin that is engageable with corresponding recesses formed in two arms secured to the respective mold halves. The mechanism must ordinarily be periodically serviced because of a tendency to wear, and, additionally, the mechanism is believed to be somewhat more complicated than is necessary to accomplish its purpose.
It will be appreciated from the foregoing that there is a definite need for a simplified molding apparatus that can be used to produce video discs, without being too susceptible to wear and damage from abrasion, and wihtout undue reliance on precise tolerances for proper operation. The present invention fulfills this need.